A single frequency network (SFN) is a broadcast network where several radio frequency (RF) transmitters concurrently send the same signal over the same frequency channel. One goal of SFNs is efficient utilization of the radio spectrum, allowing a higher number of programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to the MFN, since the total received signal strength may increase to positions midway between transmitters.
Broadcasting from two or more nearby RF transmitters on the same frequency can lead to reception problems in the overlap areas—the areas in which the RF signal level from multiple transmitters is similar in strength. Broadcasting the same signal from two or more RF transmitters on the same frequency can produce a cascade of effects. For instance, listeners located closer to any one of the transmitters where the signal is significantly stronger (usually 6 dB or greater) may hear only the closer transmitter due to the “capture effect” of a receiver. Listeners located in “equal signal” areas or fringe areas can experience serious reception problems where transmitter footprints overlap.
For SFN to work effectively, the broadcast signal from each transmitter should arrive at the receiver at the same time. A signal leaving the control site can be subject to three delay factors: control to transmitter site network link path delay, the RF “flight time” in the air from the transmitter to the receiver, and the encode/decode delay of the source program material. For an effective SFN system, the delay factors must be either compensated for or well controlled.